The invention relates to a turntable device for centering, magnetically clamping and in an operation position rotating a flat round information disc about an axis of rotation. The disc is provided with a central hub having magnetically attractable portions, a substantially cylindrical outer wall, and a central centering hole. The device includes a turntable having a turntable bottom which extends transversely of the axis of rotation and having a coaxial cylindrical turntable wall, which turntable bottom and turntable wall define a locating space for engagement with the hub of the information disc. A coaxial centering member is centered in the locating space and engageable in the centering hole of the information disc. The centering member has a free end with a pre-centering surface adapted to cooperate with the wall of the centering hole of the information disc in a not yet fully centered condition, and magnetic clamping means on the bottom of the turntable which are adapted to cooperate with the magnetically attractable portions of the hub via an air gap.
A turntable device of the type intended above is described in the Applicant's earlier European Patent Applications 0,117,576 A1 and 0,118,146 A1 (U.S. application Ser. No. 490,801 and U.S. Pat. No. 4,510,508, respectively, herewith incorporated by reference). The turntable device is capable of providing excellent centering and clamping of an information disc without any further means, such as a disc-pressure means, being needed. By accurately machining the centering member--which may be the end portion of a motor spindle--and accurately machining the center hole in the hub the currently used machining techniques enable such an accurate fit to be realised that the stringent centricity requirements imposed by modern information-disc systems, such as systems operating with optical information discs, can be met. Both the hub and turntable device need have only a small axial dimension. It is important that the mounting height of an apparatus comprising the turntable device is not enlarged unnecessarily. The turntable devices and the associated information discs of the type described are suitable for systems in which the information disc is contained in a cassette and in which the information disc must be rotatable inside the cassette housing. It is often required that the cassette can be inserted through a slot-like opening in the front of the apparatus, after which the drive means and the information disc must be made to cooperate with each other. This requires a small axial displacement of the information disc and the turntable device towards each other. The use of a hub on the information disc has the advantage that the distance between the information disc inside the cassette housing and the turntable device is already bridged at least partly by the hub.
A disadvantage of said and other known turntable devices is that a correct cooperation between the information disc and the turntable device is not obtained unless the user has already brought the information disc in a comparatively accurately central position above the turntable before these are axially moved towards each other. This is because the comparatively large axial magnetic force exerted between the hub of the information disc and the turntable gives rise to normal forces which are so large that considerable friction occurs, which may impede a further radial movement of the information disc to a central position.